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The TF39-GE-1, installed on the C-5A, was the first high-bypass turbofan engine. High bypass turbofans, meaning bypass ratios in the range of 5 to 9, power virtually all transports designed to cruise at high subsonic speeds. High bypass ratio engines provide increased takeoff thrust, low environmental noise, and low specific fuel consumption. The development of the first high bypass ratio turbofans, the TF39 for the C-5A and the JT9D for the Boeing 747, required nearly doubling the cycle pressure ratio from the 12:1 of the JT3/J79 series of jets, and increasing the turbine inlet temperature.

The TF39 engine had its component performance technology frozen in 1965 to allow for the development of an engine. Four TF39 turbofan engines power the big C-5, rated at 43,000 pounds thrust each. They weigh 7,900 pounds (3,555 kilograms) each and have an air intake diameter of more than 8.5 feet (2.6 meters). Each engine pod is nearly 27 feet long (8.2 meters).

The Lockheed C-5A Galaxy, a long-range military heavy transport, made its first flight on Jun 30, 1968. On Sep 30, 1965, the Air Force had selected Lockheed to develop and produce the heavy logistics transport aircraft. Intended primarily as a freighter, the aircraft's maximum takeoff weight was 728,000 pounds; its design payload, 220,000 pounds. On Dec 17, 1969, the Lockheed C-5A transport was formerly turned over to the U.S. Air Force during ceremonies at Marietta, Ga., where the aircraft was manufactured.

The question was whether to stick with the old and somewhat less desirable turbofans or lean on advanced engine technology and introduce a calculated risk for the sake of a superior airplane. At that time AFSC's propulsion laboratory had also performed extensive research on low specific fuel consumption engines. On the basis of in-house research, ASD planners were able to present their case for a new engine to the then Secretary of Defense Robert S. McNamara, without however, deciding precisely which new engine to procure.

The decision was to employ a new model that incorporated principles from advanced technology. It would bleed off compressor air to cool the turbine blades. It would operate with thermodynamic temperatures in the neighborhood of 2300 degrees and metal temperatures of 1500 to 1600 degrees, attaining a bypass ratio of 6 to 8 compared to current bypass ratios of approximately 2.5. By achieving the high bypass ratio and the high inlet temperatures necessary for low specific fuel consumption, C-5A designers were able to achieve the cost-effectiveness level that would make the C-5A a feasible intertheater transport. By turning to the advanced engine technology, the Air Force took a calculated risk, albeit one based solidly upon excellent research by ASD'S propulsion engineers. Their efforts in advanced technology paid off directly in the establishment of the C-5A program.

The TF39, which powers the C5A&B cargo aircraft, is closely related to the CF6 commercial family of engines and the LM2500 engine. All three are manufactured by General Electric. The LM2500, a ship propulsion version of the TF39 engine, is used to power Navy cruisers, frigates, and destroyers. The LM2500 is actually a marine/industrial version of the military TF39 aircraft engine. 3The LM2500 engine is used throughout the oil and gas industry to supply mechanical power for pumps, compressors, and generators.

The TF39 engine manufacturer, General Electric, and other sources of repair stated that the CF6 family of engines are derivatives of the military TF39 engine. The CF6 engine powers the DC-10, Boeing 747, and MD-11 aircraft. As of 1997 there were approximately 400 CF6-6 engines, 4,000 CF6-50 engines, and 1,000 CF6-80 engines in commercial aviation. The CF6-6 engine was the first commercial derivative of the military TF39 engine and is most like the military engine. There were 665 TF39 engines in DOD's inventory in 1997.

The LM2500 gas turbine engine is directly derived from GE's CF6 family of commercial aircraft engines and GE's TF39 military engine. It is in service in a large variety of vessels in the US and abroad as well as in industrial applications. In operation, the exhaust of the LM2500's high pressure turbine passes through a low pressure turbine (the power turbine) which extracts the work required to drive the main reduction gear and ultimately a driven unit, such as a ship's propellers for example. This latter operation is basically what distinguishes the LM2500 from its aircraft relative. In general terms, the aircraft turbine version expresses its output in thrust while the aeroderivative gas turbine outputs its work through a drive shaft.

The major difference in the two engines is in their bypass fans. The TF39 engine uses a two-stage bypass fan and the CF6-6 a single-stage fan. Except for the bypass fans, the other sections of the engine are very similar. While the compressor/rotor is the only identical section in both engines, according to the manufacturer, the engines share an overall 30 percent commonality among parts in the compressor, high pressure turbine, and low pressure turbine sections. Further, the repair processes and artisan skills necessary to repair the TF39 and CF6 engines are the same. For example, metal spraying, grinding and vertical turret lay work are required in the overhaul of both engines.

One way in which the DOT Volpe Center supports the transportation enterprise is by applying knowledge and practices in use by one segment of the transportation community to benefit customers in another segment of the community. For example, the Center is working with the Air Force Headquarters Air Mobility Command (AMC) by defining improvements to logistics and maintenance practices related to improved performance of the strategic airlift fleet.

In mid-1997 a Volpe Center team completed and delivered a report to AMC analyzing and diagnosing performance problems the Air Force has been experiencing with the TF39 jet engine that is used to power the C-5 aircraft. The report concluded that the TF39 is inherently a reliable jet engine and that performance problems have been due to the maintenance and overhaul practices presently used. The team's principal recommendation to the Air Force was to transition from the existing practice of repairing engines when they break down, to adopting reliability-centered maintenance precepts. This approach and related best practices are applied by the aviation industry to the maintenance of the commercial equivalent engine. The team gathered substantial data from various commercial air carriers and the engine manufacturer in support of their recommendations.

Work on the TF39 engine was previously performed in a public depot and reported as depot-level workload under the two-level maintenance concept. However, an Air Force policy change transitioned the TF39 engine to a three-level maintenance concept and reclassified the former San Antonio work as an intermediate-level workload. Further, this policy change and workload reclassification from depot-level to intermediate-level enabled the Air Force to drop this work from its workload distribution report.

By 2000 the Air Force had made significant progress to stop the decline in engine readiness. Improved engine funding, engine life management planning, and better partnering with vendors contributed to slow but steady readiness improvements in most of the Air Force engine fleet. The TF39 engine was undergoing a high-pressure turbine modification which greatly improves reliability.

The C-5 required re-engining for three main reasons. The first was to eliminate the greatest source of non-mission capable rate for the C-5. The second reason was to enable the C-5 to meet Key Performance Parameters (KPPs) defined in the ORD. Finally, re-engining would provide the C-5 with an engine that would be technologically maintainable for its projected operational life. According to the ORD, the aircraft required a higher thrust propulsion system than that currently achieved with the TF39 system.

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